Conventionally, a resin container in the shape of a bottle obtained by subjecting a polyester resin such as polyethylene terephthalate to injection molding or compression molding to form a preform, and then molding the preform in the shape of a bottle by the biaxial stretch blow molding has generally been used in wide ranges of fields as a container for beverages that accommodate various types of beverages.
In a blow molding die used in the biaxial stretch blow molding, a space (hereinafter referred to as the “cavity”) that suits to a desired shape of a bottle is formed by subjecting a material such as an aluminum alloy to cutting, followed by further cutting by means of a ball end mill. In general, in the final stage of the cutting, the material is cut by means of a ball end mill having a diameter of 2 mm with a feed pitch of about 0.05 mm, whereby the height of cutting marks formed by a ball end mill on the surface of the cavity (hereinafter referred to as the “scallop height”) is allowed to be about 0.3 μm. Then, after the cutting, the cavity surface of the blow molding die is mirror-finished by polishing (about Rz 0.01 stipulated by JIS B0601). By conducting biaxial stretch blow molding by using the blow molding die, a resin container having luster and transparency is obtained. However, if matting or less transparency is required for the outer surface of a resin container for the reason of design, a surface treatment or the like (e.g. sand blasting) may be further conducted in order to impart the outer surface with roughness.
In recent years, with the aim of lowering the production cost, decreasing the amount of wastes or the like, there has been a growing demand for thinning a resin container in order to decrease the amount of raw material resins used. However, if the resin container becomes thin, the strength of the container is inevitably decreased, and as a result, the container tends to be deformed easily. In particular, in the case of an angular resin container, when a blow molding die is opened to release the container, a side wall part 41 that has a larger area as compared with a corner surface part is stuck to the die surface, and the side wall part is pulled outwardly. As a result, as shown in FIG. 4, a problem arises that a corner surface part 42 that has been released from the mold earlier than the side wall part 41 acts such that it deforms in the form of a recess towards the inside of the container. This phenomenon becomes significant in biaxial stretch blow molding in which a die is heated to a high temperature (the crystallization temperature) at the time of biaxial stretch molding. For example, in Patent Document 1 or the like, in order to prevent deformation that occurs at the time of mold releasing associated with thinning of a container, an angular resin container is proposed in which the cavity surface after the cutting is allowed to be roughened.
In the blow molding, a resin container is molded by setting a heated preform in a blow molding die and blowing a high-pressure air. If the cavity surface of the blow molding die is a mirror-finished surface, a problem arises that air remains between the container and the cavity surface, and as a result, pockmark-like air pool marks are formed on the surface of the container, resulting in deterioration of the appearance. Under such circumstances, Patent Document 2 or the like propose a blow molding die in which, by blasting sand to the surface of a cavity after the cutting to allow the surface to be stain-like rough surface, a gap is formed between a resin to be stretched and the cavity surface, whereby air is released.